D.B.
I don't think kids at this age have the coordination to handle roller blades. In addition to the helmet, you need to get knee, elbow and wrist guards too (essential to prevent those broken bones you speak of!), and most kids even at 7 find these a hassle to put on. I doubt a 4 year old would enjoy this. At 4, most kids still don't have the coordination to ride a bike - they are still negotiating with training wheels. For a 4 year old, I would try something else - even a scooter with a brake would be easier for him, assuming he wouldn't go too fast. Don't rush him - you'll be glad you waited.